Assessment of Uncertainty in Volume Estimation of Non-Static Target: A Phantom Study using Racemosa Wood

INTRODUCTION

CT information of the target undergoing motion/movement during its scanning has been questioned by many researchers for its preciseness as well as accuracy. The present study was taken with aim to validate the racimosa wood as lung equivalent and to assess the uncertainty in volume estimation during virtual simulation of non-static target of known dimension such as in lung cancer radiotherapy.

MATERIALS AND METHODS

The racemosa wood was validated as lung equivalent material with the help of two methods. Wood insert with tumor model was put into the hollow cylinder space of dimension 6.4 cm in diameter provided in CIRS phantom. First CT image of rest position was taken and given name "No Movement". Subsequently the tumor was shifted +/-5mm, +/-15mm and +/-25 mm with respect to "Rest Position". CT images of the CIRS phantom containing tumor in wood cylinder were acquired after each movement given to wood cylinder.

RESULTS

The relative electron density of racemosa wood corresponding to HU value -724 was found to be 0.275 gm/cm3. The true volume of the target was 7.8.cm3 however variation up to 9.5 cm3 was observed in CT produced volume of the target over the range of different movements.

DISCUSSION

The racemosa wood was found to be having range of density (- 850 HU to - 400 HU) similar to real human lung density variation. Various studies have been performed using uniform density lung structures in their experimental setups to assess the accuracy in lung cancer radiation delivery. However, in the present work approximately real clinical setting was reproduced by putting the wood cylinder with density variation from 0.2 gm/cm3- 4.5 gm/cm3in hollow space provided in one lung structure the phantom used in this study.

CONCLUSIONS

The racemosa wood was found to be lung equivalent which is available locally and cost-effective as well. Overestimation in the target volume (by CT imaging) showed a trend of increase with 3 directional movement amplitudes. The results of this study can be utilised in lung cancer radiotherapy as the same were derived from setup having clinical settings in terms of lung density variation, shape, compositions of the phantom maximally as found during the real patient radiotherapy.

Validation of the Gel & Wax Boluses and Comparison of their Dosimetric Performance with Virtual Bolus

Background

In general, radiotherapy treatment planning is performed using the virtual bolus. It is necessary to investigate physical bolus in comparison to virtual one.

Objectives

In the present study, first, radiological properties of superflab Gel bolus and Paraffin wax bolus was investigated in terms of their relative electron density. Then, dosimetric performance of both the bolus (i.e. Gel and Parafin wax) was compared with Virtual bolus.

Material and Methods

In This experimental study, the radiological property of Wax and Gel boluses was investigated using two methods. In one, the relative electron density of both the Gel and Wax boluses was calculated by measuring their linear attenuation coefficient where in another method relative electron density was calculated by recording their CT No directly from their CT scan. Later CT scan of solid water slab phantom (dimension 30x30x15 cm³), with physical boluses (i.e. Gel and Wax bolus) of appropriate thicknesses required to deliver a dose of 200 cGy at D_max using 4 MV, 6 MV and 15 MV photon beams, was taken. These CT data sets were retrieved to TPS. A plan was done to deliver a dose of 200 cGy at D_max using Single 4 MV, 6 MV and 15 MV photon beams. Dose at depths D_max, 1 cm, 2 cm, 3 cm, 4 cm and 5 cm was recorded. Using this similar method, doses at depths viz D_max, 1 cm, 2 cm, 3 cm, 4 cm and 5 cm was recorded for the Gel and Wax boluses. The differences in dose of gel and wax bolus from virtual bolus were recorded for comparison of their dosimetric performance.

Results

The measured (calculated) relative electron density of wax and Gel bolus was found to be 0.958 (0.926) and 0.923 (0.907), respectively. Variation in dosimetric performance of Gel and Wax with reference to Virtual bolus was studied. However, on average, Gel bolus was more consistent with virtual bolus.

Conclusion

To avoid any dose difference between, delivered (using physical bolus) and planned (using virtual bolus), the physical boluses should be investigated for their dosimetric performance in comparison to virtual bolus. The results obtained and methodology used in this study can be applied in routine radiotherapy practices.

Lymphovenous obstruction in anomalous midgut rotation

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